(a) Technical Field
The present disclosure relates to a method for removing residual water in a fuel cell, in which the humidity of purge gases is controlled to effectively remove residual water in the fuel cell and to maintain the humidity of the membrane at a nearly constant level, thus ensuring enhanced durability of the membrane.
(b) Background Art
In general, a polymer electrolyte fuel cell (PEFC) comprises a fuel cell stack in which a plurality of unit cells are stacked. In each unit cell, an anode and a cathode are disposed on both sides of an electrolyte membrane to form a membrane electrode assembly (MEA), and the MEA is disposed between separators (bipolar plates).
In the fuel cell, hydrogen as fuel is supplied to the anode (“fuel electrode”) and oxygen in air is supplied to the cathode (“air electrode” or “oxygen electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions and electrons by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are transmitted to the cathode through a gas diffusion layer (GDL) and the bipolar plate.
At the cathode, the hydrogen ions supplied through the electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen in the air supplied to the cathode to produce water.
The electrochemical reaction occurring in the fuel cell is affected by various factors including the surface area of the catalyst layer in which the reaction occurs, the used hydrogen, the adhesion between the oxygen electrode and the electrolyte membrane, the reaction temperature of the electrodes, and the pressure of reactant gases. Also, the generated current is affected by the factors.
Condensed water and impurities generated at each electrode reduce the active surface area of the catalyst layer to cause a loss to the reaction, thus deteriorating the performance of the fuel cell. Accordingly, the condensed water and impurities generated at each electrode in the fuel cell should be removed properly.
For these reasons, a working fluid discharge apparatus has been adopted to remove the condensed water and impurities generated at each electrode in the fuel cell. FIG. 1 is a schematic diagram showing a conventional working fluid discharge apparatus for a fuel cell stack.
Referring to FIG. 1, hydrogen containing gas is supplied from a hydrogen tank 12 to an anode 10 through a fuel processing system (FPS) 14, which processes fuel to be dissociated into hydrogen and increases the content of hydrogen.
On the other hand, outside air, i.e., oxygen containing gas is supplied to a cathode 20 through an air filter 22, a silencer 24, an air blower 26, and a humidifier 28.
At this time, the hydrogen ions, electrons and oxygen react to produce condensed water and impurities at the anode 10 and the cathode 20.
The condensed water and impurities generated at the anode 10 are discharged to the outside when a purge valve 32 is opened under the control of a fuel cell system controller 30. That is, hydrogen purging (discharging) is periodically performed to remove the condensed water and impurities generated at the anode 10 of the fuel cell stack, thus maintaining the performance of the fuel cell stack.
Meanwhile, U.S. Pat. No. 7,132,179 (B2) discloses a method for reducing water content in a fuel cell by controlling the humidity of reactant gases by a water balance calculation. Especially, a threshold value below which the stack performance is reduced is set to a critical membrane moisture level such that the water content is not reduced below the threshold value. However, it takes several hours to reach the threshold value, and its applicability is not satisfactory.
In addition, U.S. Pat. No. 6,358,637 discloses a method for removing residual water in a fuel cell using a vacuum pump after a fuel cell system is shut down, which is effective in removing residual water using the vacuum pump when the temperature of the fuel cell is high; however, it requires a significant amount of energy to remove the water content in the fuel cell using the vacuum pump.
Moreover, U.S. Pat. No. 6,864,000 discloses a method for shutting down a fuel cell system including a plurality of fuel cells arranged in a stack, in which the fuel cells are cooled to a shutdown temperature while maintaining a substantially uniform water vapor pressure through the fuel cells whereby migration of water within the fuel cells during cooling is reduced. However, this method is still not satisfactory.
As discussed above, the prior art methods cannot effectively remove water from the fuel cell to maintain the humidity of the membrane. That is, it may take a long time to remove water and additional power may be required to operate a blower or vacuum pump for removing the water. Additionally, conventional purge methods result in dryout of the membrane, which is known to promote degradation of the membrane.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.